It is well-established in aerodynamics that boundary layer transition from a laminar flow to a turbulent flow leads to large increases in skin friction on an aerodynamic surface. Therefore, delaying a boundary layer's laminar-to-turbulent transition (or “BLT” as it is known in the art) can produce a significant reduction in the overall drag experienced by an aerodynamic surface.
In the subsonic or incompressible boundary layer case, there are streamwise or planar Tollmien-Schlichting (TS) instability waves as well as oblique TS instability waves where “obliqueness” is viewed relative to the streamwise direction. However, in the subsonic or incompressible boundary layer regime, only streamwise or planar TS instability waves are of concern since they are the ones that first amplify sufficiently to cause BLT. In the presence of free stream turbulence or discrete-roughness-elements, one observes narrow, streamwise elongated regions of alternating high and low streamwise velocity in the boundary layer. These elongated regions are known in the art as “streaks”. As is also known in the art, streaks stabilize the unstable and streamwise TS waves and weakly oblique TS waves when the wavelengths of the TS waves are between four and five times that of the streak wavelength.
In the supersonic and hypersonic regimes, the mechanism causing BLT is very different as the most amplified (or dominant) instabilities correspond to highly oblique first-mode waves for supersonic and planar/axisymmetric Mack mode waves for hypersonic. Furthermore, the compressible boundary layer always exhibits a generalized inflection point that induces the inviscid instability characteristic to the instability waves that is not present in favorable to weakly-adverse pressure gradient incompressible boundary layers. At supersonic speeds, which correspond to Mach numbers higher than 1 but roughly lower than 5, the generalized inflection point of the compressible boundary layer leads to the highly obliqueness of the first mode waves. At hypersonic speeds, which correspond to Mach numbers roughly higher than 5, a region of relative supersonic flow induces the acoustic mode characteristic to the inviscid Mack mode waves. Although the dominant inviscid Mack-mode waves in hypersonic flow are planar as the viscous TS waves in the subsonic flow, they are fundamentally different because of the physical mechanisms that lead to their exponential growth. As is known in the art of BLT in a supersonic boundary layer, the BLT process involving oblique first-mode instabilities is characterized by a strong amplification of stationary streamwise streaks, and further, that artificial suppression of such streaks has been shown to delay the onset of BLT. In a hypersonic boundary layer, the BLT process involving planar/axisymmetric Mack-mode instabilities is characterized by the non-linear saturation of the Mack-mode waves and subsequent three-dimensional secondary instability that quickly lead to the onset of BLT.